Radio frequency identification tag ("RFID tag") arrangement systems have been used to detect and identify objects, animals, persons, etc. In the case of numerous objects, such as documents in an office or similar setting, such systems have not had much applicability. As an example, there are some 100 billion documents generated each year in the US, and many of these documents are stored in filing cabinets, drawers, shelves, etc., with no readily automated way to locate specific documents.
Existing document locating systems are generally manually operated. For example, common office document locating systems include alpha numeric labeling and computer based indexing systems. Bar code label systems, often used in conjunction with color-coded tabs and/or alpha-numeric labels, have enjoyed increased use as a document locating system over the past decade. Office files labeled with bar codes are typically located using a hand held optical scanner. Such bar code labeling systems utilize a light beam emitted from the scanner to "read" the bar code label. These systems require a direct line of sight between the scanner and the bar-code label, thus greatly limiting their utility.
File labeling based on bar codes, alpha numeric characters, or color codes require placing a file in a pre-designated location. If these files are not placed in pre-designated locations, then these filing systems do not automatically identify that a file is misplaced or lost. With bar codes, alpha numeric, and/or color coded labeling systems, the user must first search for the file in its pre-designated location before determining that the file is missing. Upon discovering that a file is missing, these systems are unable to locate the missing file.
The retrieval/replacement of misplaced or lost files and/or documents is costly and time consuming. In offices, warehouses, or other facilities having large volumes of files, significant time and energy are frequently expended searching for missing or lost files. In most instances, it is imperative that a file be quickly located. In a recent study, Gartner Group showed that (1) between 3% and 5% of files in a typical office in the US are either missing (location unknown) or lost, (2) the typical cost of locating a missing file is $100, (3) the typical cost of replacing a lost file is $500, and (4) an average US executive spends around $10,000 a year searching for missing files. The time expended to locate office documents strongly influences office productivity.
Radio frequency (RF) identification systems utilizing RFID tags are known in the art and are often used to identify the presence or location of certain objects. With such systems, either RF, light or sonic waves are illuminated onto tags attached to objects. After receiving the incident signal, the identified tag emits a response, from which the system determines the location of the responding tag. RFID systems typically consist of one or more transceivers (exciters) and one or more tags, with each tag attached to an object whose location is desired to be identified. RFID systems have been used for identifying vehicles, animals, parcels, laundry, people, railway cars, inventory in warehouses, golf balls, and the like.
An RFID tag is an electronic device that generally incorporates a specific and typically unique identification number, where the number may be "read" by an interrogating RF transceiver (transmitter/receiver) system. The tag is generally attached to an object so that the object's presence or location in a given area may be identified by an interrogating RF transceiver system which "reads" the tag's identification number. Since the communication to the tagged object is by RF energy, such systems do not require direct line-of-sight between the transceiver and the tagged object and the tagged object may be located within a closed box, cabinet, or drawer.
RF tags, sometimes described as transponders, may be active (powered by a battery) or passive (acquiring energy from the incident radio frequency field). Passive tags, such as disclosed in U.S. Pat. Nos. 4,654,658 and 4,730,188, have fewer components, are smaller in size, and generally less expensive than active tags. In order to collect sufficient energy to operate, passive tags are located typically from one centimeter up to one meter from the transceiver.
RF tags typically consist of an antenna or a coil, to collect RF energy (from which the tag derives it's operating power), and an integrated circuit (IC) which contains an identification code or other information in its on-chip memory. Attaching a tag to an object enables the object to be located with the aid of an RF interrogation system. When the transceiver transmits a coded radio frequency signal, nearby tags collect energy from the transceiver's RF field. If the tag's ID number is the same as that encoded in the incident RF field, then the tag is activated by the incident RF field and, in response, modulates the incident RF field.
Commercially available passive RF tags generally operate at low frequencies, typically below 1 MHz. Low frequency tags usually employ a multi-turn coil resulting in a tag having a thickness much greater than a standard sheet of paper or a standard file folder, thereby rendering low frequency tags unsuitable as "smart labels" on paper documents. However, high frequency passive RF tags, operating around 2.45 GHz, and typically consisting of a single turn, flat antenna, printed onto a flat single layer sheet of plastic or paper are thin and thus, their thinness renders them suitable as a smart label for tracking paper documents. These high frequency tags are being targeted for use on labels in such applications as tracking fresh food packages, clothes in laundries, garments in hospitals, baggage in airports, and the like. It is believed that these high frequency tags have not yet been used for tracking documents in office settings.
There are several possible reasons why high frequency RF tags have not been targeted for tracking office documents. One possible reason why high frequency tags have not enjoyed much success in this field may be because high frequency RF fields are readily absorbed by moisture typically found in office documents. Also, office files are commonly stored in metal cabinets and, since RF fields do not readily transmit through metal, achieving transmission of RF fields into and out of a metal cabinet is perceived to be a difficult task. Further, a large, multi-office organization may have hundreds, thousands, or even millions of files stored in numerous, distant locations, which may be located throughout many different sites. The magnitude of the transmitted RF field decreases rapidly as the distance increases from the scanner. Since the tag needs a finite amount of RF energy to become activated, there is a maximum distance, typically one or two meters, beyond which a passive tag collects insufficient RF energy to activate its on chip circuitry. To have practical value in a commercial setting, it is believed that the user should be able to locate a tagged document from a distance of at least five meters between the users' PC and the tagged document.
Several conventional RFD tag systems will now be described. PFID tag systems generally consist of a personal computer (PC) or other computing device, a radio frequency transmitter which sends an RF signal to the tag and which "excites" the tag into generating an RF response, and a receiver which receives the excited response from the tag. Such combined transmitter/receiver units are described, for example, in U.S. Pat. Nos. 5,537,105 and 5,550,547.
A conventional RFID tag system architecture is illustrated in FIG. 1 and includes PC 2, transceiver (transmitter/receiver unit) 4, and passive tag 6. The communication link between PC 2 and transceiver 4 may be via hard wiring, RF, or optical link. Transceiver 4 transmits an RF signal to tag 6, which excites tag 6. Transceiver 4 then receives a response from tag 6, which is transmitted to PC 2 for identifying the characteristics of tag 6. Examples of prior art RFD systems employing this direct communication between the transceiver and the tag are disclosed in U.S. Pat. Nos. 5,537,105 and 5,550,547.
In other conventional RFD systems, it is necessary to provide a separate transceiver unit at each location from which the tags are to be monitored. As described in U.S. Pat. No. 4,703,327, a passive interrogator label system is frequently configured such that a plurality of tags are interrogated from a number of different locations. For example, if persons with RFID tags are authorized to enter a building, several transmitting and receiving antennas are normally placed near different doors to the building to identify the RFID tags. Another example is where the RFID tags are placed on cattle which are monitored at different locations, such as a holding area, a feeding area, etc. Further, the RFID tags may be placed on railroad cars to permit car identification at different locations. As the number of locations increases, the equipment requirements and costs also increase significantly.
FIG. 2 is an illustration of another conventional RFID system. As illustrated in FIG. 2, such a conventional RFID system consists of host transceiver 12, a plurality of local transceivers 8, and a plurality of tags 10 a, b, c . . . n. In some instances, PC 2 controls or exchanges data with host transceiver 12. Again, the communication link between PC 2 and host transceiver 12 may be via hard-wiring, via RF, or via an optical link. The plurality of local transceivers 8 and the host transceiver 12 generally each include a transmitter and a receiver, such as are known in the art. Each tag 10 a, b, c, . . . n, which is attached to the object to be located, such as an animal, person, box, etc., contains a unique, preprogrammed identification number. A digitized RF signal in which the unique ID number is encoded is transmitted, at a first frequency f.sub.1, from the host transceiver 12 to the plurality of local transceivers 8. Local transceivers 8 in turn transmit, at a second frequency f.sub.2, the received RF signal to the plurality of tags 10 a, b, c, . . . n. A particular tag 10 (e.g., 10c) that is within the transmitted range and having the associated identification number will respond by modulating the second frequency f.sub.2. The modulated RF field f.sub.2 is detected by the receiver portion of local transceiver 8, thereby identifying the excited tag. The local transceiver then transmits, at a third frequency f.sub.3, to host transceiver 12, which in turn identifies the tag data with the aid of PC 2. The identification and location of the excited tag can be determined because each local transceiver 8 has a unique identification number, and PC 2 and host transceiver 12 can address each local transceiver 8 uniquely and sequentially.
The system described above herein is termed "two way" because the communication path is bi-directional, or two way, from host transceiver 12 to local transceiver 8, from local transceiver 8 to tags 10, then back from tags 10 to local transceiver 8, then from local transceiver 8 to host transceiver 12. Local transceivers 8 in the "two way" RF system incorporate one receiver which receives a signal of the first frequency f.sub.1 from host 12, one transmitter which transmits a signal of the second frequency f.sub.2 exciting tags 10, one receiver which receives the modulated response signal of the second frequency f.sub.2 from an excited tag 10, and one transmitter which transmits a signal at the third frequency f.sub.3 back to host 12. Similarly, host 12 has one transmitter which transmits a signal at the first frequency f.sub.1 to the local and one receiver which receives a signal of the third frequency f.sub.3 from local transceiver 8. In the most general case first frequency f.sub.1, second frequency f.sub.2, and third frequency f.sub.3 are different frequencies. In a simpler, more practical case, first frequency f.sub.1 and third frequency f.sub.3 are the same, while second frequency f.sub.2 is different.
The performance, complexity, and cost of such two-way systems, however, have limited utility and applicability in an office environment. The architectures of existing two-way RFID systems, described in FIGS. 1 and 2 above, for example, would require a very large number of transmitters and receivers to implement an automatic document tracking system in a commercial office. Specifically, the architecture of an RFID system shown in FIG. 2 and described above would require three transmitters and three receivers. The large number of transmitters and receivers employed in such an architecture would render the cost of an automatic document locating system impractical and prohibitive in essentially all applications. There is a widespread demand for a cost effective, automatic document location system for commercial offices and, at present, it is believed that no such system exists.